This invention relates generally to the field of lasers, and more particularly, is directed to a semiconductor laser device having a current confinement structure and a built-in waveguide structure.
With the increased use in recent years of optical disk devices, such as digital audio discs, video discs, and "document files," and the spread of fiber optic communications, mass production of semiconductor lasers, which provide the light source of these applications, has become essential. In the manufacture of conventional semiconductor lasers, a liquid phase epitaxial process (referred to hereafter as "LPE") is used for the crystal growth on the substrate. The LPE process, however, is not suitable for mass production of semiconductor lasers for the following reasons. First, the LPE process may not be used with a large size semiconductor substrate. Second, the process is not suitable for sufficiently controlling crystal growth on the substrate. Finally, the process requires a substantial amount of time to complete. For these reasons, other processes for crystal growth, such as molecular beam epitaxy (referred to hereafter as "MBE") and metalorganic chemical vapor deposition (referred to hereafter as "MOCVD"), have been used for mass production of semiconductor lasers.
A semiconductor laser having a suitable construction for manufacture by the MOCVD process is disclosed in Applied Physics Letters, Vol. 37, No. 3, p. 262 and is illustrated in FIG. 1. As shown in FIG. 1, current confining layer 5 which confines the current to the direction parallel to the junction plane and controls the transverse mode of the laser, is provided in upper clad layer 4 formed on flat active layer 3. The active layer 3 is formed from a different conductive type material than neighboring p-type clad layer 4 and p-type coating layer 6. Thus, no current flow passes through layer 5. Active layer 3 is formed on an n-GaAs substrate through an n-GaAlAs clad layer 2 and p-GaAlAs clad layer 4 is covered with a p-GaAs contact layer. Further, the p-GaAs contact layer and the n-GaAs substrate have a pair of metal electrodes 7, 8, respectively.
Current injection to active layer 3 is restricted to a stripe shape groove portion wherein current confinging layer 5 is partially removed. Also, the thickness of clad layer 4 is so thin that wave guided light in active layer 3 is capable of spreading to current confinging layer 5 and coating layer 6. Therefore, the propagation constant of the vertical mode to the junction plane just beneath the strip is different than the propagation constant of the vertical mode to the side portion of the stripe. It can be considered that the tranverse mode, which is horizontal to the junction plane, is wave guided in accordance with the distribution of the effective refractive index. The effective refractive index is given as the propagation constant of the vertical mode to the junction plane divided by the propagation constant of light in a vacuum. Thus, a wave guided mode is generated just beneath the stripe. This type of laser device produces both a current confinement effect in the horizontal direction of the junction plane by confining the current in layer 5 and a built in refraction index waveguide effect. Thus, it is reported that a low threshold, of around 50 mA, is achieved with room temperature pulse oscillation and that single mode oscillation is achievable so that the transverse mode can be controlled satisfactorily.
It is very important for semiconductor lasers to have a low threshold not only to improve their life span and to reduce operating current, but also to improve the performance of the laser. Buried hetero structures (BH) and transverse junction stripe structures (TJS) are typical examples of conventional lasers having a low threshold value of 10 to 20 mA or less. In contrast, the threshold value of a laser with the construction shown in FIG. 1 is 50 mA which is more than double the value for lasers with BH and TJS structures. Tests conducted by the inventors showed that with present structures, it is difficulet to reduce the threshold value. The reason for the difference between the laser illustrated in FIG. 1 and lasers having a BH or TJS structure is probably due to differences in the waveguide effects in the structures.
With the structure of FIG. 1, light guided by active layer 3 infiltrates through the clad layer 4 to current confining layer 5 and is subjected to absorption. Thus, differences in the imaginary parts of the equivalent complex refractive index are produced in the horizontal direction at the junction plane, thereby giving an absorption loss guide in which light is guided.
In the case of a BH or TJS structure, the real part of the complex refractive index is affected by infiltration of light wherein light is guided through differences in the real parts, i.e., refraction indexes. Accordingly, the threshold value is probably increased by an amount corresponding to the absorption loss in the structure shown in FIG. 1.
Considering the above mentioned drawbacks of waveguide loss structures, it is better to make refractive index structure lasers which do not have a high absorption loss. A semiconductor laser of this type is shown in FIG. 2. This device basically includes the same layers as in the device shown in FIG. 1, however, clad layer 4 is thick enough not to infiltrate light to current confining layer 5 which has a high refractive index and absorbs lights. Coating layer 6, which has a higher refractive index than clad layer 4 and does not absorb light, is provided at the stripe shape groove portion.
In this structure, light guided in active layer 3 infiltrates to coating layer 6, which has a higher refractive index just beneath the stripe, and to clad layer 4, which has a lower refractive index at the sides of the stripe. Thus, there are produced differences of effective refractive indexes of the traverse mode to the vertical direction of the junction plane between the inside and outside of the stripe, making a waveguide by the refractive index waveguide effect so as to confine light only beneath the stripe.
It has been found, however, that lasers manufactured in accordance with FIG. 2 often fail to achieve a low threshold value. The reason for this condition is believed to be the actual structure itself. It has been found that in the structure of FIG. 2, there is considerable leakage of laser light into coating layer 6 at the sides of the stripe shape groove portion. This situation will be explained in more detail with reference to FIG. 3. FIG. 3 illustrates calculated curves indicating the relationship between effective refractive index Neff of the lowest waveguide mode in the light waveguide path and refractive index Nc of the higher refractive layer provided at one side of the light waveguide path in the typical double-hetero structure laser. This mathematical model corresponds to the portion of the structure just beneath the stripe of the laser shown in FIG. 2. FIG. 3 indicates that the effective refractive index Neff gradually increases when the refractive index Nc of coating layer 6 is increased with a fixed thickness of clad layer 4. Note, however, that the increase in Neff is rather small compared to the increase in Nc. Thus, Neff&gt;Nc is true only when Nc is relatively small and Neff=Nc at a certain larger value of Nc. If Nc is beyond this larger value, Neff is smaller than Nc. This means that the outside of the light wave guided path has a larger value in refractive index thus causing infiltration of light to the outside and therefore, no waveguide mode exists. In other words, the waveguide mode is cut off when Neff=Nc. Thus, the refractive index of clad layer 4 should be at a minimum, a value which does not cause "cut off", and a maximum value where .DELTA.Neff=Neff-Neff.degree. (Neff.degree. is the effective refractive index when there is no coating layer 6) by providing coating layer 6. FIG. 3 also indicates that the increase of the effective refractive index depends on the thickness of clad layer 4. The smaller the thickness, the larger the increase in effective refractive index.
In designing the ideal waveguide portion of the laser structure shown in FIG. 2, the following should be considered. First, the .DELTA.Neff should be larger than 10.sup.-3 in order to stabilized control of the traverse mode in the horizontal direction of the junction plane. Second, thickness h of clad layer 4 should be as large as possible in order to obtain a higher manufacturing yield and to minimize damaged to the active layer when the groove portion is etched adjacent the active layer. It is clear that these ideal considerations are inconsistent. For example, if 0.3 .mu.m is selected as the thickness of clad layer 4, which is practically easy to realize, the refractive index of coating layer 6 should be very near to cut off in order to realize .DELTA.Neff=10.sup.-3 -10.sup.-2 in accordance with FIG. 3.
For a laser having the structure shown FIG. 2, it is necessary to establish structural parameters near cutoff of the waveguide mode. This causes the lasing threshold value to be higher than expected due to the following problems. The first problem is that the lasing threshold is apt to become the condition of cutoff because of variations in the thickness and refractive index in active layer 3, clad layer 4 and coating layer 6. Another problem is that the cut off condition of the waveguide mode depends upon the thickness of clap layer 4. Thus, the waveguide mode often becomes cut off where thickness h is larger even if there exists a waveguide mode where thickness h is smaller. In the structure shown in FIG. 2, even if it is designed to be waveguided in the flat portion just beneath the stripe shape groove, the sides of the stripe corresponds to the portion wherein thickness h of clad layer 4 becomes substantially larger and thus it becomes a cutoff condition for the waveguide mode. This phenomenon is also made clear by the fact that radiant brightnesses appear at the sides of the stripe on top of the active layer. This fact is revealed in infrared microscope examintion of this type of laser.
In a laser with the structure of FIG. 2, the threshold current is expected to be lower than in a laser with the structure of FIG. 1 due to the change from a loss waveguide structure to a refraction index waveguide structure. However, it is not always possible to obtain a lower threshold because of the above mentioned new losses.